Both sites offer simple "microblogging," but Posterous is focused on getting people to post by email. Posterous also launched "group blogging" today. (Tumblr, meanwhile, is in the process of preparing "pro" features that will, in theory, start bringing in revenue.)

Tumblr dwarfs Posterous in traffic: Compete says 1.1 million unique U.S. visitors went to Tumblr.com last month, while Posterous.com drew only 120,000. This isn't the best metric, as both sites include other ways to access/post content, but it should be directionally correct.